1. Field of the Invention
The present invention relates to a high-frequency oscillator, and more particularly, relates to an injection-locked high-frequency oscillator, which is increased in its stability of oscillating frequency.
2. Description of the Related Art
A high-frequency oscillator having oscillating frequency of approximately 1 to 100 GHz has been mostly used as an oscillating source of a radio communication device, an optical cable transmission device or a measuring apparatus. Oscillating type of the high-frequency oscillator can be categorized into several different types, one of which is that of a so-called ring oscillator, in which an oscillation closed loop is formed by connecting a plurality of oscillating amplifiers in a transmission line of a high-frequency.
FIG. 1 is schematic view illustrating this type of conventional high-frequency oscillator.
This high-frequency oscillator is provided with transmission line 1 for high frequency signals, and first to third amplifiers 2a to 2c for oscillation. Transmission line 1 has a microstrip line structure in, for example, an unbalanced mode, and comprises a annular signal line provided on one principal plane of substrate 1 formed of, for example, a dielectric material, and a grounded conductor (not shown) provided on a substantially entire surface of the other principal plane of substrate 1. Through transmission line 1, an electric field generating between signal line 4 and the grounded conductor and the accompanying magnetic field transmit high frequency waves, i.e., electromagnetic waves, and resultantly a high-frequency electric current flows through signal line 4.
First to third oscillating amplifiers 2a to 2c are inserted in transmission line 1 so that respective of them have input terminals, respectively, directed in an identical direction to annular signal line 4 in one of the principal planes of substrate 3. Namely, signal line 4 is cut at three positions, and the input and output terminals of each of the oscillating amplifiers are respectively connected to opposite ends of each of the cut positions. Thus, signal line 4 (i.e., transmission line 1) and first to third oscillating amplifiers 2a to 2c form the oscillation closed loop.
Upon viewing from an arbitrary point of the oscillation closed loop, the oscillating frequency f0 of the described high-frequency oscillator becomes a frequency that is a positive feedback containing a delay amount (time delay) of oscillating amplifiers 2a to 2c, namely, the feedback with an identical phase, and corresponds to wavelength xcex. In conclusion, oscillating frequency f0 is basically determined by the electric length of a line, containing the delay time of the oscillating amplifiers. In other words, the length of transmission line forming the oscillation closed loop is set at a length that becomes a positive feedback with a desired oscillating frequency with consideration of delay amounts of oscillating amplifiers 2a to 2c. The amplification degree of respective oscillating amplifiers 2a to 2c must be 1 or more at the oscillating frequency, containing a transmission loss. At this stage, the output of the oscillating frequency is derived from any one of the oscillating amplifiers, for example, oscillating amplifier 2a by the microstrip line.
Nevertheless, in the high-frequency oscillator of the above-described constitution, since the oscillating frequency is almost determined by the electric length of transmission line 1, such problems have been encountered that the Q factor (degree of acuteness of resonance) of the oscillator is small, and the frequency stability is low. Therefore, it has been tried that a synchronizing signal from high stability signal source 5 such as a quartz-crystal oscillator, is injected to, for example, the input side of oscillating amplifier 2b. Here, the frequency of the synchronizing signal is set at a value of one third of the frequency f0 of the oscillator, i.e. f0/3. As a result, as shown in FIGS. 2A and 2B, assuming that the wavelength corresponding to the oscillating frequency f0 is xcex, the phase at every 3xcex is urged to be equalized by the synchronizing signal, so that the stability of the oscillating frequency of the high-frequency oscillator is increased. This type of high-frequency oscillator is called as an injection-locked high-frequency oscillator. It should be noted that FIG. 2A illustrates the waveform distribution of a component of the oscillating frequency f0 along transmission line 1, and FIG. 2 illustrates the waveform distribution of the synchronizing signal f0/3 along transmission line 1.
Nevertheless, in the case of the injection-locked high-frequency oscillator, since the frequency of the synchronizing signal from signal source 5 such as the crystal oscillator cannot be more than the upper limit of 500 to 600 Hz in the present state of affairs even if an overtone oscillation or a multiplying circuit is employed. Thus, the higher is the oscillating frequency f0 (in GHz band) of a high-frequency oscillator, the longer the interval of injection locking of this high-frequency oscillator becomes. Accordingly, there occurs a problem such that the phase of the high frequency cannot be equalized within the interval of the injection locking while causing jitter or fluctuation thereof, to result in reduction in the stability of the oscillating frequency.
An object of the present invention is to provide an injection-locked high-frequency oscillator, capable of equalizing the phases of oscillating frequency components thereby increasing the stability of its oscillating frequency.
The object of the present invention can be achieved by an injection-locked high-frequency oscillator in which an oscillating frequency thereof is determined by an electric line length of an oscillation closed loop, the high-frequency oscillator comprising, an annular transmission line, m units of oscillating amplifiers disposed in the transmission line, m being defined as an integer larger than 1, and means for injecting synchronizing signals each having a frequency of 1/mn of the oscillating frequency into in-phase points in the oscillating closed loop, n being defined as a natural number, wherein the transmission line and the oscillating amplifiers form the oscillation closed loop, and wherein when a wavelength corresponding to the oscillating frequency is defined as xcex, an electric line length from any one of the oscillating amplifiers to the neighboring oscillating amplifier is set as nxcex by taking into consideration a delay time due to the oscillating amplifiers.
In the present invention, the m units of oscillating amplifiers are disposed in the annular transmission line to form the oscillation closed loop, and the electric line length from any one of the oscillating amplifiers containing the equivalent electric line length of the one oscillating amplifier to the neighboring oscillating amplifier is set as nxcex. Further, in the oscillation closed loop, the synchronizing signals of 1/mn of the oscillating frequency f0 are injected into the in-phase points. As a result, the phases of the oscillating frequency f0 are equalized to thereby increase the stability of the oscillating frequency. The in-phase points are typically disposed between every two neighboring oscillating amplifiers.
A further detailed description of the injection-locked high-frequency oscillator of the present invention will be provided below. In this description, the frequency of the synchronizing signal is defined as f5, and the number of multiplication of the synchronizing signal f5 against the oscillating frequency f0 is defined as being xcex1. Namely, the equation xcex1=mn=f0/f5 is established.
When n=1, xcex1=m. In this case, the intervals between the respective injection points of the synchronizing signals in the transmission line become one wavelength (xcex) of oscillating frequency f0. Further, at the respective injection points of the synchronizing signals, i.e., the in-phase points of the oscillating frequency, xe2x80x9cmxe2x80x9d numbers of peaks of the waveform of oscillating frequency f0 pass during one cycle (m/f0) of the synchronizing signals. More specifically, the waveform corresponding to the length of mxcex passes. Therefore, at the respective injection points, a component of oscillating frequency f0 is drawn into the synchronizing signals at every mxcex to equalize the phases.
When the injection points of the synchronizing signals at xe2x80x9cmxe2x80x9d positions in the transmission line are equidistantly disposed, namely when the substantial electric lengths of the transmission line including the oscillating amplifiers are identical to one another among respective neighboring injection points, the synchronizing signals are injected into the oscillating frequency at every injection point in such a manner that respective phases are shifted from one another by an amount of xcex (=2xcfx80). Therefore, during one circulation of a high frequency signal around the oscillation closed loop, the phases are equalized by the synchronizing signals at every wavelength xcex of the oscillating frequency.
Although the above description was provided under such a condition that n=1, and the number m of the oscillating amplifiers is equal to the number xcex1 of multiplication, if the number xcex1 of multiplication is 2m, the phases of the oscillating frequency will be equalized at every 2xcex under the above-described condition. Further, in the case where the intervals between every neighboring injection points where the injection of the synchronizing signal is respectively conducted are determined so as to correspond to two wavelengths of the oscillating frequency, equalizing of the phase will be conducted at every 2xcex of the oscillating frequency.